A Ni-based superalloy substrate for use in a blade for jet engines, gas turbines or the like is strictly required to have high oxidation resistance and corrosion resistance. Such required high-temperature oxidation resistance has been obtained through a surface diffusion treatment, for example, by coating a substrate surface with an Al2O3 film. For covering the insufficient performance of this treatment, there has also been developed a technique for forming a diffusion barrier layer of Pt or the like on a substrate. Rhenium (Re) can be used as the diffusion barrier layer to provide enhanced high-temperature corrosion resistance. Re excellent in thermal shock resistance is also used as high-temperature members or components of various combustors, such as a rocket-engine combustor, or high-temperature nozzles. Heretofore, there have been known the following processes for forming a Re-based film or a Re alloy film.
(1) Sputtering Process or Physical Deposition Process
A physical deposition process allows a film thickness and/or composition to be readily controlled. On the other hand, it involves problems, such as, (i) many restrictions on the size and shape of a substrate, (ii) the need for a large-scaled apparatus and complicated operations and (iii) a relatively large number of defects or cracks in an obtained film.
(2) Thermal Spraying Process
A thermal spraying process involves problems, such as, (i) a relatively large number of defects in an obtained film, (ii) lack of compatibility to the formation of thin films (10 μm or less) and (iii) poor process yield and low economical efficiency.
(3) Re-Alloy Electroplating Process
There have been known a Ni—Cr—Re alloy film having a Re content of up to 50 weight % (this percentage becomes lower when converted into atomic composition ratio), a Ni—Co—Re alloy film (see, for example, Japanese Patent Laid-Open Publication Nos. 09-302495 and 09-302496), and a Re—Ni alloy film for electric contacts, which has a Re content of up to 85 weight % (63 atomic %) (see, for example, Japanese Patent Laid-Open Publication No. 54-93453). In all of the above plated films, the content of Re is in a low level.